The present invention relates to an apparatus and to a method for making fine metal powder, and more particularly relate to such an apparatus and method for making metal powder of which the diameters of the particles are on the order of some few hundreds of angstroms.
The present invention was originally made in Japan, and the first patent application made thereof was Japanese patent application No. 8/536/83, of which priority is being claimed in the present application; and the subject matter of that previous Japanese patent application is hereby incorporated into this specification by reference; a copy is appended to this specification.
In the past, fine metal powder, such as the type of metal powder used for sintering material and as dispersion material for making particle dispersion composite materials, has generally been made by the method of mechanically pulverizing solid metal, by the method of atomizing molten metal, or by the method of colliding a stream of molten metal with an object at low temperature; but the diameters of the particles of metal powder made by such prior art methods as above have typically been of the order to form ten to five hundred microns.
In general, the smaller are the diameters of the particles of a fine metal powder, the better is the metal powder for use as raw material for sintering or for making particle dispersion composite materials, because in the case of sintering the higher the density of the resultant sintered material becomes, and in the case of making a particle dispersion composite material the better the mechanical properties of the composite material become, due to the increase in the total surface area of the particles of the metal powder relative to their total weight, which increases the relative importance of their surface activity. Therefore, it has been realized for a long time that it is very desirable to make fine metal powder with as small a particle diameter as possible, and energetic efforts have been expended with this aim in view.
One method that has been experimented with for making very fine metal particles is called the vacuum vapor deposition method. In this method, a metal is heated in vacuum and is vaporized into gas composed of its atoms, and this gas is then condensed on the surface of a low temperature object. Another method that has been attempted involves vaporizing a metal in a low pressure but not vacuum environment consisting of an inert gas at a pressure of from a tenth to a hundredth of atmospheric pressure or so, so that the vapor of the metal is cooled by the inert gas so as to be brought into the oversaturated state, and condenses into fine powder in either the liquid or the solid phase. This method is called the gas vaporization method, and small amounts of fine metal powder have been produced on an experimental basis in this way.
These methods have been successful in making fine metal powder with average particle diameter less than one micron, but, since all of these methods make use of gradual vapor condensation phenomena, there is a large fluctuation in the particle diameters of the metal powder obtained (i.e. the standard deviation of these diameters is great), and futhermore the rate of production of metal powder is extremely low. In order to improve the productivity of these methods, it is necessary continuously to take out the generated metal vapor from the chamber in which it is generated, and to cool it. Therefore, there have been proposed methods in which the metal vapor is carried on a plasma flow to take it out of the metallic vapor production chamber, and is then cooled by striking it or colliding it against a water cooled copper plate. Also, methods have been proposed in which the metal vapor is absorbed into a sheet of oil which is dripping down, and again is condensed in this way. However, the former method involving the use of a water cooled plate for condensing the metal vapor requires large and expensive facilities, while the latter method of absorption into oil is not good in absorption effiency. Accordingly, in the prior art, it has been difficult to mass produce fine metal powder with very small and uniform particle diameter in an efficient and economical way.
A subsidiary problem that has been realized with the manufacture of fine metal powder is that, when the particle diameters are very small, and when the powder is manufactured in vacuum conditions or in an atmosphere composed of inert gas, the powder may have a tendency towards self ignition when it is removed and is brought into contact with ordinary atmosphere, even at normal temperatures. This is because, as the particle diameter decreases, the surface area of the particles included in a given mass of metal powder increases dramatically, and therefore the activity of the particles increases. Therefore, in the past, it has been recognized to be desirable to perform post processing of fine metal powder before removing it into the atmosphere from vacuum or an inert atmosphere where it has been formed, by forming an oxide film on the surfaces of the particles under controlled conditions. However, according to such conventional methods, this has increased the cost of the process, as well as lowering the quality of the finished product.